1. Field of the Invention
The present invention relates to an improved fibre optic probe, or sensor, for remote flow measurements. In particular, this sensor is designed for accurate flow measurements of fluids flowing in remote vessels, such as blood flow within arteries or veins or flows within pipes.
2. Background Information
Fibre-optic anemometry is employed in velocimetry to measure flow rates, velocity gradients, and turbulence at remote points which are otherwise inaccessible. For example, by measuring the velocity of blood flow in an artery before, during, and after an angioplasty procedure, the success of the procedure can be ascertained. Laser light is transmitted, via optic fibres, into the flow where it is scattered. A portion of the scattered light is collected and transmitted, also via optic fibres, to an anemometer for analysis. By analyzing the Doppler shift between the transmitted light and the collected scattered light, the velocity of fluid flow can be ascertained.
Optical fibres were first used in laser Doppler anemometers for the measurement of localized blood flow velocities by T. Tanaka and G. B. Benedek and described in an article entitled Measurement of the Velocity of Blood Flow (In Vivo) Using Fibre Optic Catheter and Optical Mixing Spectroscopy, 14 Applied Optics 189-196 (1975). In their system they used a 500 .mu.m core diameter monofibre to deliver the laser beam into the femoral vein of a rabbit. The immersed distal end of the fibre was cut and polished at 30.degree. relative to the fibre axis in an attempt to minimize flow disturbance caused by the mere presence of the fibre in the blood stream. A laser beam was projected out through the fibre wall, opposite the cut end surface, into the flow by total internal reflection at the angled polished distal end of the fibre. Light scattered by the erythrocytes at the fibre tip was collected by the same fibre and mixed with the reference beam on the surface of a photomultiplier tube. Analysis of the resultant signal was done on an 18-channel digital autocorrelator.
The sensor of the Tanaka-Benedek system suffers from a number of disadvantages. Projection of light out of the side of the fibre necessitates that the fibre be stripped to its core, thus leaving the brittle and fragile fibre core exposed and unprotected. Cutting and polishing the distal end of the fibre is a difficult operation to perform, thus causing manufacturing complications. Finally, due to the small radius of curvature of the exposed fibre, the curved outer surface of the fibre could cause most of the light scattered back to the fibre to be lost at the fibre-fluid interface, especially if there are irregularities on the surface.
R. B. Dyott, in an article entitled The Fibre-Optic Doppler Anemometer, 2 Microwaves, Optics and Acoustics 13-18 (1978), discusses making flow measurements using a single optic fibre laser Doppler anemometer with the fibre normally terminated. He reported that the region in which light is back-scattered into the fibre extends only to a few tens of the core diameter in front of the fibre tip. As demonstrated in FIG. 10, the flow in this region, indicated at 70, is perturbed by the presence of the distal end of the fibre which could seriously affect the accuracy of any measurements of flow velocity. The Dyott system is well suited, however, to measurements in situations where the medium is stationary and the particles are moving.
For flow measurements, G. A. Holloway, Jr. and D. W. Watkins modified the Tanaka-Benedek system by using separate fibres for delivering the laser beam and receiving the scattered light as described in Laser Doppler Measurement of Cutaneous Blood Flow, 69 J. Investigative Dermatology 306-309 (1977). They applied such a modified system for non-invasive measurement of cutaneous microcirculation. The disadvantages described above regarding the single fibre Tanaka-Benedek system are exacerbated by the inclusion of a second fibre.
For invasive flow measurements, D. Kilpatrick adapted Dyott's system by modifying the analyzing components and described the adapted system in Laser Fibre Optic Doppler Anemometry in the Measurement of Blood Velocities In Vivo, Computers in Cardiology, IEEE, 467-470 (1980). By using this system, he showed that, despite flow perturbation caused by the presence of the fibre, the system could still be used to measure blood flow velocities both in vitro and in vivo. With the fibre positioned parallel to fluid flow he obtained a broad spectrum, declining monotonically with width, that is proportional to the flow velocity. The maximum Doppler shift frequency was taken as representative of the flow velocity and this agreed with the calculated theoretical value of 4.2 Mhz/ms.sup.-1 (i.e. the maximum shift frequency is absolutely calibrated). A linear relationship was obtained between the maximum shift frequencies and the flow velocities for flows of up to 1.5 ms.sup.-1 in the forward direction (advancing towards the fibre tip) but only 20 cms.sup.-1 for flows in the reverse direction (moving away from the fibre distal end tip).
Concurrently with the work of Kilpatrick described above, M. Imamura, F. Kajiya, and N. Hoki independently developed a similar system, but with an added advantage of being able to measure directional flow, as reported in Blood Velocity Measurement By Laser Doppler Velocimetry With Optical Fibre, Proc. 12th Int. Conf. Med. and Biol. Eng. 35 (1979). They achieved this by using a Bragg cell (acousto-optic modulator) to shift the reference signal by 40 MHz. In vivo flow measurements in blood were made via the fibre's distal end and with the whole fibre oriented at a 60.degree. angle to the flow (see FIG. 11), and a broad rectangular spectrum was obtained. A linear relationship was again found between the maximum shift frequencies and flow velocities as in the Kilpatrick adaptation of the Dyott system.
It is interesting to note the absolutely calibrated linear relationship between the maximum Doppler shift frequency and flow velocity obtained for the Kilpatrick and Imamura-Kajiya-Hoki systems described above. This relationship implies that single fibre systems measure the free stream flow velocity (i.e, velocity outside the perturbed region), but only within certain velocity limits. Outside these limits, however, the system will either have to be modified or improved to allow an accurate measurement of flow velocity. The broad spectrum observed by both systems was assumed to be due to multiple frequency shifts from the particles of varying velocity in the perturbed region at the tip of the fibre.
The slight difference in spectral shape reported by the Kilpatrick and the Imamura-Kajiya-Hoki studies is due to the area of turbulence at the measurement region adjacent the fibres' distal end, and this has been theoretically addressed by M. D. Stern in Laser Doppler Velocimetry in Blood and Multiply Scattering Fluids: Theory, 24 Applied Optics 1968-1986 (1985). The difference was attributed to different thicknesses of the boundary layer at the distal end tip of the fibre, with Kilpatrick's system having a thicker layer. To overcome the effect of the boundary layer for obtaining accurate flow measurements, it is necessary to project the probe volume (i.e., the volume in which flow measurements are made) away from or beyond the boundary layer and into the laminar flow region. To do this Stern suggested use of two fibres, with one fibre delivering the incident light and the other collecting the scattered light. The sensor proposed by Stern, however, projected the probe volume from the blunt ends of the fibres.
A two fibre laser Doppler anemometer with the fibres oriented at 60.degree. to the direction of flow was developed, tested, and reported by Y. Ogasawara, O. Hiramatsu, K. Mito, and others in A New Laser Doppler Velocimeter With a Dual Fibre Pickup For Disturbed Flow Velocity Measurement, Circulation, 76, Suppl. 4, 328 (1987) and by F. Kajiya, O. Hiramatsu, Y. Ogasawara, and others in Dual-Fibre Laser Doppler Velocimeter and its Application to the Measurements of Coronary Blood Velocity, 25 Biorheology 227-235 (1988). In both systems, two step-index fibres with a core diameter of 50 .mu.m and a cladding diameter of 62.5 .mu.m were used. The scattered light collected by the receiving fibre was mixed with the reference beam and detected using an avalanche photodiode. The spectrum analyzer showed a narrow spectrum (as compared with the single fibre system) with a peak value that varied with flow velocity. The separation between the cores of the two fibres in these systems was 12.5 .mu.m.
By varying the core separation and using different fibre combinations, S. C. Tjin, D. Kilpatrick, O. Hiramatsu, Y. Ogasawara, and F. A. Kajiya obtained better linearity between the Doppler frequencies and flow velocities as the core separation was increased with their system and findings described in A Dual-Fibre Laser Doppler Anemometer for in Vitro Measurements, Proc. 13th Aust. Conf. Optical Fibre Technology, 245-248 (1988). This improved linearity, however, was obtained at the expense of a decreased signal-to-noise ratio, and the probe volume was still projected from the distal end of the fibre.
However, with a fibre probe placed parallel to the flow, S. C. Tjin, D. Kilpatrick, and P. R. Johnston found that a two-fibre probe with the fibre tips normally terminated is inadequate for accurate flow measurements, especially for flows moving away from the fibre tips, as described in Evaluation of the Two-Fibre Laser Doppler Anemometer for In Vivo Blood Flow Measurements, Experimental and Flow Simulation Results, 34 Optical Engineering, 460-469 (1995). This is because the flow at the fibre distal end tips is perturbed, and the region of perturbation extends away from the fibre tips with increasing flow velocity. For flow towards the fibre tips, the region of flow perturbation decreases towards the fibre tips with increasing flow velocity. These changes in the region of flow perturbation with flow velocities and the direction of flow give rise to a non-linear calibration between the Doppler frequency and the flow velocity. This limits the usefulness of the system for in vivo flow measurements because, in most practical systems, the fibre optic probe must be placed parallel to the flow.
A two fibre sensor adapted to project a probe volume to the side of the catheter wall by means of reflective surfaces was proposed by S. C. Tjin in Fibre Optic Laser Doppler Anemometry, Ph.D. Thesis, University of Tasmania, 1991, available at the University of Tasmania. Such a sensor was, however, never constructed. In the proposed embodiment of the sensor, two fibres are embedded in the wall of a larger catheter. Proximate each fibre distal end tip, a separate opening is formed in the catheter sidewall. An angled reflective surface is positioned in the opening axially opposite the fibre distal end tip to reflect light from the fibre radially outwardly through the opening directly into the flow, which is parallel to the fibre axes. This proposed embodiment, if built, would have had a number of disadvantages. The uncovered openings at the reflective surfaces would themselves cause turbulence and would also provide a place for blood clots to form or collect. To minimize the size of the openings, and thus the amount of turbulence caused thereby, it was proposed that two small, circumferentially spaced apart openings be provided in the catheter wall rather than a single large opening and single reflective surface that would be able to accommodate both fibres. Polishing, mounting, and aligning dual reflective surfaces, however, would introduce manufacturing complexity and alignment problems to developing a suitable probe volume and would add cost to the manufacture of the sensor. Also, the embodiment, as proposed, included no provision for focusing the transmitted and received light beams to minimize the width of the Doppler spectrum and maximize the signal-to-noise ratio.